The Captive Network Assistant appears because your machine attempts to visit the page http://www.apple.com/library/test/success.html.
If it is successful, the Captive Network Assistant doesn't load.
Since you're on a captive portal, you're probably redirecting all traffic to your captive portal, so your machine can't reach the page in questions & thus launches the Captive Network Assistant.
In order to avoid it from launching, you can rename it so that it isn't found, however that only applies for you & your machine. How would you take care of it for multiple users? Surely you're not going to rename everyone's Captive Network Assistant?
If you need to do this for multiple users, you'll need to modify your captive portal ACLs to either allow HTTP access to www.apple.com or create a redirect that leads www.apple.com to point to a host that is reachable via captive portal & contains the folders & file its looking for.
I don't have any problems w/ the Captive Network Assistant on iDevices (iPads, iPods, iPhones), however the Captive Network Assistant in Mac OS X 10.8.x seems to be very limited in functionality & doesn't seem to want to run the Java applet that ExpressConnect requires, nor does it seem to allow me to download the network profile needed for a manual installation. Safari, on the other hand, allows me to do both.
Wondering if anyone knows just how limited the Captive Network Assistant actually is?